It is probably a combination of things. Try removing the formatting in the widget or in the item definition. Instead of {{'%.1f' | sprintf:itemValue(config.max_temp_actual)}} just leave {{itemValue(config.max_temp_actual)}}.
Maybe you have alredy noticed that the formatting problem also occurs with the set point. I can see in the image of your post that under the “Set point” combo, the set point temperature has a duplicated ºC.
It probably has to do with the binding itself, I cannot tell you right now.
This is the new code line, i also removed the "C°" from line
<td class="text-left"><strong>{{itemValue(config.max_temp_actual)}}</strong></td>
And in the setpoint line i also removed "C°"
<div class="col-xs-6"><span style="color: red; font-size: 16pt">{{itemValue(config.max_temp_set)}}<small style="font-size:8pt"></small></span></div>
Like Achim I burned the CUL firmware to my MAX-Cube, so I have a similar chain: Homegear –-> Homematic --> Openhab. For me to make this widget work, I had to rename the control mode options, next to the removal of '%.1f' | sprintf:
My options needed to be:
AUTOMATIC --> AUTO-MODE
MANUAL --> MANU-MODE
BOOST --> BOOST-MODE
VACATION --> PARTY-MODE (??)
Not sure why these are different and if it has anything to do with the CUL firmware.